368 



PHOTOSYNTHESIS 



Chlorophyll {a,b) 



with hot 

 alkali 



with acid 



► (Iso-) chlorophyll 

 iajy) 



with acid with hot 

 »Phseophytin(t;, /O alkali 



Phytochlorin e 



Phytorhodin g 



The manipulations involved in the hydrolysis and saponification reac- 

 tions and the separation of the products by fractionation of the ether 

 solutions with hydrochloric acid must be carried out with great care. 



Another portion of the original ether solution containing all the leaf 

 pigments is used for the determination of carotin and xanthophyll. This 

 is saponified with methyl alcoholic potash to remove the chlorophyll, the 

 yellow pigments remain in the ether. These are transferred to petroleum 

 ether solution and the xanthophyll removed with methyl alcohol ; the 

 carotin remains in the ether. 



The quantities of the four pigments are determined colorimetrically 

 by comparison with standard solutions. For the determination of the 

 chlorophyll components known quantities of different pure chlorophyll 

 preparations, e.g., crystalline ethyl chlorophyllid or the methyl ph^eophor- 

 bids are used and these are treated in exactly the same manner as the 

 extracts of unknown concentration ; the solutions finally obtained are 

 then compared in a colorimeter. The standard solutions for the deter- 

 mination of carotin and xanthophyll are prepared from pure crystalline 

 preparations of these pigments or solutions of potassium bichromate can 

 also be used. In the hands of a skillful manipulator the method of 

 Willstatter and Stoll can doubtless yield valuable comparative results. 

 It need, however, hardly be emphasized that considerable experience 

 in handling solutions of the pigments and in the various chemical reac- 

 tions involved is an absolute requisite for the successful operation of the 

 method. 



, The method has probably not the accuracy of spectrophotometric meas- 

 urements. Schertz ^^ has shown that the spectrophotometric method for 

 xanthophyll has an accuracy of 2.8 per cent while the colorimetric method 

 has an accuracy of 17 per cent. The same author has also described 

 results of the si:«ctrophotometric determination of carotin. 



The cliloropJiyU-content of leaz'cs, as determined by the method of 

 Willstatter and Stoll, is fairly constant in normal leaves: 0.15 to 0.35 

 gram per 100 grams fresh weight, 0.6 to 1.2 gram per 100 grams dry 

 weight and 0.3 to 0.7 gram per one square meter of leaf surface. The 

 same authors found that for one mole of chlorophyll h there were present, 

 on an average, 2.9 moles of chlorophyll a. On the basis of dry weight, 



Schertz, Jour. Agri. Res., 26, 383 (1924); 30, 254 (1925). 



